It has been happening to me in Chrome on my Android phone, but not Chrome on the PC browsers.
Sometimes it works on the phone, but it is only about 1 out of 3 attempts. It has only been happening to me for about the past week.

Okay, that seems to match the serial / fingerprint of the intermediate certificate I’ve got so either the root certificate is not being sent with the rest of the chain or it’s corrupted somehow.

Given that people are only experiencing this on certain browsers/systems, what’s weird is that it works fine on my Android 6 phone with Chrome. I’m guessing the site isn’t supplying the root certificate and some devices don’t already have it in their Trusted Certificates store on their device.

Without having a device where I get this error, it’s harder to diagnose further but thank you for the help @danielccm!

I like your enthusiasm! Unfortunately it gets a lot more convoluted for little extra gain and ultimately, we can’t fix the error.

Well, you might be able if it is what I think it is but telling people to install certificates into their trusted certificate store would be dead against ethics and having people listen to that kind of advice from strangers is a sure fire way to get their devices horribly compromised, so let’s not

If anyone tells you to “just add this certificate into your trusted certificate store”, politely tell them to go and pound sand.